In optical communication or optical information processing, an optical waveguide device is used in which an optical waveguide is formed on a substrate having electro-optic effect as lithium niobate (LN) or the like.
In an LN modulator which is an example of the optical waveguide device, a shape of a waveguide is in a Mach-Zehnder (MZ) structure. A modulator having a MZ structure controls on/off of light by an applied voltage which is applied to a control electrode arranged along a branched optical waveguide of a MZ structure. Specifically, when an output waveguide of the waveguide having a MZ structure is a single mode waveguide, on/off of the light is able to be performed by fundamental mode and excited mode control due to a change in a propagation speed of the light according to the applied voltage.
Properties of the LN modulator include Vπ and an optical bandwidth. Vπ is an applied voltage necessary for turning the light off-state from on-state, and the optical bandwidth is a frequency at which an on/off operation is able to be performed. As to properties of the LN modulator, it is preferable that Vπ is small and the optical bandwidth is wide.
In general, when a voltage is applied to a capacitor parallel circuit, a large electric field is applied to a capacitor side having large dielectric constant. As disclosed in Patent Literature No. 1, when a thickness of the substrate of the LN modulator becomes thin to the extent of a light distribution, a microwave electric field efficiently overlap the optical waveguide, and thus it is possible to drive the LN modulator at low Vπ.
However, when the substrate is thin, the optical characteristics are easily degraded (unstable) compared to an optical modulator including a thick substrate. One of the reasons is light leakage propagating inside the substrate except for the optical waveguide. For this reason, a countermeasure against the light leakage is necessary for the thinned LN modulator as disclosed in Patent Literature No. 2.
Recently, a method of modulating a phase such as a tetravalent phase modulation (Phase Shift Keying, PSK) type optical modulator (4 level PSK modulator) has been mainly used. In order to modulate the phase, it is necessary that a plurality of the optical waveguides having a MZ structure is arranged. For example, when the tetravalent PSK modulator is used, three MZ structures are necessary, and when a polarization multiplexing tetravalent PSK modulator is used, six MZ structures are necessary.
Specifically, as illustrated in FIG. 1, in the tetravalent PSK modulator, it is necessary that input light is branched into four light rays. In FIG. 1, a nest type waveguide is used in which two small MZ structures (sub MZ) are arranged in branched waveguides of one large MZ structure (a main MZ). Each separated waveguide is subjected to phase modulation in its interacting portion. A coplanar waveguide structure is applied to an electrode for modulation corresponding to each MZ structure.
When considering crosstalk of electrical signal among MZ structures and a GND function, it is necessary that a GND (ground) electrode width between MZ structures of the tetravalent PSK modulator corresponding to the closest distance between the sub MZ structures of FIG. 1 is at least 200 μm. For this reason, a separation gap (a Y branch gap) of an incident Y branch of light four-branched portion is 200 μM or more.
In Non Patent Literature No. 1, although a tetravalent PSK modulator using InP which is able to be more downsized than the LN modulator is used, the Y branch gap of 350 μm is ensured in consideration of the crosstalk of the electrical signal and the GND function.
In addition, a wide angle Y branch in which a branch angle of a Y branch is great is considered even in integration of an optical circuit. An object of Non Patent Literature No. 2 or Patent Literature No. 3 is to reduce a loss in the Y branch portion, and all influences of the light leaked from the Y branch portion are not considered.
On the other hand, a processing method of the light leaked from the optical waveguide is disclosed in Patent Literature No. 4 or 5, and in particular, a technology relevant to light leakage from a curved portion or a Y-shaped multiplexing portion of the optical waveguide is disclosed. In addition, the optical waveguide in which light leakage occurs from an S-shaped portion has wavelength dependency, and thus is not able to be typically used as an optical device in a communication field.
As described above, in the optical waveguide device such as the tetravalent PSK modulator, the separation gap of the incident Y branch of the light four-branched portion is wide, but a length of the Y branch is limited by a constraint of a size of the optical waveguide device, and thus a Y branch angle is greater than normal. As a result thereof, the light is leaked from the Y branch portion, and thus the optical characteristics of the optical modulator or the like are degraded. Further, when a thinning structure is used in which the substrate is thinned, as described above, the optical characteristics are remarkably degraded.